Large CT colonography trial opens with mixed reactions

A large multicenter trial of CT colonography has started collecting patients and some criticism among advocates who say it may undercut the procedure and produce not much more than is known already.

The American College of Radiology Imaging Network is projected to enroll more than 2300 patients at 15 sites across the U.S. during a one-year accrual period. ACRIN 6664 is expected to be the world's largest virtual colonoscopy study. But that may not be enough for some practitioners.

"While I applaud the broad goals of the ACRIN trial, it seems that we are once again starting from scratch and in effect reinventing the wheel," said Dr. Mark E. Klein, a radiologist at Washington Radiology Associates in Washington, DC who already performs virtual colonoscopy.

Dr. Peter B. Cotton and Dr. Perry J. Pickhardt had reached opposing conclusions in two previously published large multisite studies of the value of CT colonography. Cotton, director of the Digestive Disease Center at the Medical University of South Carolina, used 2D primary reads and found virtual colonoscopy to be significantly less sensitive than conventional colonoscopy (JAMA 2004;291[14]:1713-1719). Pickhardt, an associate professor of radiology at the University of Wisconsin, Madison, used 3D primary reads and reported that virtual colonoscopy had a high sensitivity, comparing favorably with optical colonoscopy for detecting significant polyps (NEJM 2003; 349[23]:2191-2200).

Smaller studies have also produced conflicting data. A carefully controlled study that will provide the final word on CT colonography is needed, according to Dr. C. Daniel Johnson, a professor of radiology at the Mayo Clinic in Rochester, MN, and principal investigator for the ACRIN trial.

Klein disagrees. The Pickhardt study was well designed, well executed, and demonstrated excellent sensitivity for virtual colonoscopy polyp detection, he said. He is concerned that virtual colonoscopy's potential could be undervalued by other studies that use techniques different from the Pickhardt protocol.

Pickhardt said another trial is unnecessary. Although the Cotton study was published after his, Cotton had conducted the study much earlier and had used outdated techniques, he said.

"The two major advances that allowed for our success were primary 3D polyp detection and oral contrast for tagging," Pickhardt said.

ACRIN will use both 2D and 3D primary read protocols and will take advantage of the latest technology, Johnson said. Every site will use at least a 16-slice scanner. But questions remain about what constitutes a 3D viewing protocol, Pickhardt said.

"Virtual colonoscopy software systems are not created equal. It may be misleading to say that primary 3D detection is being employed, since most systems do not yet allow for this approach. Disappointing results could therefore be misinterpreted as a failure of 3D reading," he said.

But evaluating 2D versus 3D primary reads is only one of the trial's goals, Johnson said. Secondary aims include assessing interobserver variability, determining the importance of flat lesions and extracolonic findings, identifying reader preferences, measuring patient acceptance, and creating a database for computer-aided detection. Pickhardt was unmoved.

"For nearly a year, our virtual colonoscopy program at the University of Wisconsin has enjoyed coverage for screening virtual colonoscopy from local third-party payers. I see little reason why other groups couldn't simply follow our lead," he said.